Electro-mechanical wave motors also called ultrasonic motors are well known in the art and such motors are increasingly used due their superior properties and relatively low cost compared to other motors such as conventional electromagnetic step motors. Generally electro-mechanical wave motors are can operate with very high accuracy and without any substantially slip and simultaneously with very low noise.
EP1941608 describes a stator for a for an electro-mechanical wave motor in form of an electro-mechanical wave device comprising a substrate for propagation of a mechanical wave and for transmission of electrical signals through conductors accommodated by the substrate. A plurality of actuators are positioned on the substrate for generation of the mechanical wave and each of the actuators are connected to a respective set of conductors of the substrate for reception of an excitation signal transmitted by the set of conductors. The actuators generate the mechanical wave propagating in the substrate along a predetermined propagation path in response to the excitation signals.
US2011025167A describes an ultrasonic motor with a stator having n number of piezoelectric elements affixed to one surface of a vibrating member formed of a plate member whose outer circumferential edge is circular or polygonal. The vibrating member comprises 4n piezoelectric elements that are secured to one surface of the vibrating member for generating a traveling wave of a 3n wave (n is a natural number) traveling in a circulating manner by vibrating the vibrating member. The 4n piezoelectric elements being disposed so as to be dispersed in a circumferential direction, which is a direction, in which the traveling wave circulates,
US2011215675 discloses a stator in form of a piezoelectric oscillator that generates a travelling wave using two B (1, n) mode (n is a natural number) standing waves that are out of phase with each other by 90 degrees. On a lower surface of the oscillating body, (4/3)n piezoelectric elements are provided in order to generate an n-wave travelling wave by combining the two B (1, n) mode standing waves that are out of phase with each other by 90 degrees.
To provide a very accurate electro-mechanical wave it is generally desired that the wave formation has a substantially size to ensure an effective operative connection between the stator and the rotor.
However the inventors of the present invention have observed that when the standing waved of the stator may result in torsion of the stator in particular where the stator has a closed travelling path. This torsion is undesired for several reasons. For example the torsion may result in reduced operative connection between the stator and a rotor and further the torsion may result in undesired wear and inducement in defect in the stator during use.